1. Field of the Invention
The present invention relates generally to implantable medical electronic devices and more specifically relates to an electrode structure for recording of neural signals.
2. Description of the Prior Art
Electrical coupling of electrodes to neural tissue has been known for some time. The structure taught in U.S. Pat. Nos. 3,421,511 and 3,654,933 issued to Schwartz, et al and Hagfors, respectively, and assigned to the assignee of the present invention, are intended for chronic implantation to accomplish neural stimulation rather than neural sensing or recording. Therefore, these electrode configurations are optimized for stimulation and shielding is not employed for improvement of signal-to-noise ratio.
Stein and Pearson teach the desirability of reduced cuff diameter for neural sensing in Journal of Theoretical Biology, Vol. 32 (1971) at pages 539-558. The minimization of cuff diameter is limited, however, because of the damage caused by "pinching" the neural tissue. Similarly, damage results from wrapping wires around nerves as taught by Schad and Seller in Brain Research, Volume 100 (1975), at pages 425-430. In an attempt to overcome this problem, Ninomiya, et al, in Journal of Applied Physiology, Volume 41 (1976) at pages 111-114 teach the making of electrodes from soft cow skin. This approach, however, creates the risk of an immune reaction and is susceptible to electromagnetic interference as reported in the cited article.
An effective electrode arrangement is the tripolar configuration as taught by Testerman, et al, in Medical Research Engineering, Volume 10 (1971) at pages 6-11. See also Stein, et al, in Brain Research, Volume 128 (1977), at pages 21-38. Whereas the tripolar arrangement enhances signal-to-noise ratio, it does not provide for a traumatic minimal cuff diameter nor optimal rejection of electromagnetic interference.